This invention relates to propylene copolymers having excellent heat resistance and transparency and, at the same time, having softness.
In recent years there has been an amazingly rapid development of packaging materials and containers such as synthetic-resin films and bottles. There has been an urgent demand for improvement of the heat resistance with regard particularly to transparent and, moreover, soft materials among these articles. The utilization of materials of this character in the field of medical supplies such as transfusion liquid bags and blood bags and in the field of foodstuff packaging in forms such as squeeze bottles for mayonnaise, ketchup, and the like is being considered. For such utilization, in order for these materials to exhibit their inherent functional characteristics, they must have transparency and softness, needless to say. In addition, however, they must also possess heat resistance (i.e., the capability of retaining their shapes at high temperatures) sufficient to withstand sterilization processing such as that with hot water. Moreover, the production cost of such a material must, of course, be of an order which is reasonable with respect to its use.
As soft materials of this character, synthetic resins such as low-density polyethylenes, ethylenevinyl acetate copolymers, and soft polyvinyl chlorides have been used until now. However, the melting points of low-density polyethylenes and ethylene-vinyl acetate copolymers are a little over 110.degree. C. to approximately 90.degree. C., whereby these resins, as they are, cannot sufficiently withstand sterilization processing with hot water. Furthermore, low-density polyethylenes, which have relatively high melting points, cannot be said to have very high transparencies. While soft polyvinyl chlorides possess heat resistance for withstanding sterilization processing with hot water, they contain residual monomer and substances such as a plasticizer, which are toxic and therefore give rise to problems in the use of these resins in the fields of medical supplies and foodstuff packaging.
On one hand, among the copolymers of propylene, a random copolymer of propylene with ethylene or of propylene with butene-1 has heat resistance for withstanding hot-water sterilization, but the degree of softness is still inadequate for the uses which are considered to be problematical at present. The reason for this is that, although the copolymer tends to become a soft polymer as the content of the ethylene or butene-1 is increased, it then tends to become a rigid or hard polymer as the proportion of these comonomer units in the form of blocks is increased. Further, some of these copolymers have higher transparencies than propylene homopolymer but have the tendency to begin to have lower transparencies in a region of high comonomer content similarly as in the above described transformation into a rigid polymer. As a result, these copolymers become unsuitable for applications to the aforementioned various uses.
A copolymer of propylene with a branched .alpha.-olefin such as 4-methylpentene-1 also has good heat resistance similarly as in the case of a copolymer of propylene with ethylene or butene 1. However, in order to obtain the desired degree of softness, this expensive comonomer must be introduced in a high concentration. Furthermore, in the case where the copolymer is softened by an introduction of the comonomer in such a high concentration, also, the transparency is very poor, which is a serious problem.
It is known that a copolymer of propylene with a straight-chain .alpha.-olefin of 5 to 12 carbon atoms such as hexene-1 provides a material of excellent softness and transparency even with a comonomer content of relatively small proportion. In this connection, for this copolymer, to be considered as a soft material, it is desirable that the value of its Olsen bending stiffness be 6,000 kg/cm.sup.2 or less, preferably 5,000 kg/cm.sup.2 or less. There may be no lower limit for the bending stiffness as long as the copolymer is to be regarded as a soft material, but the copolymer having the bending stiffness of 1,000 kg/cm.sup.2 or less, especially of 500 kg/cm.sup.2 or less, may be disadvantageous from the practical point of view since the copolymer tends to be tacky or less heat-resistant.
If a copolymer having a high heat resistance which could not be attained by the already existing copolymers and fully retaining the excellent softness and transparency possessed by a copolymer of propylene with a straight-chain .alpha.-olefin having 5 to 12 carbon atoms of this character could be produced, it would be of great advantage and utility.